"Smart" materials include shape memory alloys, piezoelectric ceramics, polymer gels, and electrorheological fluids. These materials respond in some desired way to a change in temperature, pH, moisture, electric or magnetic fields, or some other factor. For example, polymer gels change size and shape in response to a stimulus and thereby convert chemical energy into mechanical energy. Colloidal suspensions change from a free-flowing liquid to a gel in response to electric or magnetic fields. Elastomeric composites change modulus (i.e., resistance to deformation) by applying a magnetic field. That is, molded elastomeric matrices having magnetic particles embedded therein increase in resistance to deformation when subjected to compressive or shear forces. Such elastomeric composites could be used to isolate or absorb vibrations.
New materials that respond intelligently to changes in temperature or moisture have also been developed. For example, polyethylene glycols bonded to various fibrous materials such as cotton and polyester possess the intelligent properties of thermal adaptability and reversible shrinkage. Thermal adaptability involves imparting a "thermal memory" to a material such that when the temperature rises, the material cools off, whereas when the temperature drops, the material warms up. Such thermally adaptable materials could be used for biomedical products for body-temperature regulation and burn therapy. Reversible shrinkage involves imparting a "dimensional memory" to the material such that when the material is exposed to a liquid (e.g., water) it shrinks in area. Such materials could be used for pressure bandages that contract when exposed to blood, for example, thereby putting pressure on a wound.
Polymers and polymer surfaces have been modified to change their characteristics in response to external stimuli, such as pH. Such responsive surfaces could be important for imparting adhesion, printability, coatability, and biocompatibility to plastics. Memory shape polymers have been widely used in the packaging industry. Most of these materials are crosslinked semicrystalline materials that when heated shrink or deform from a metastable state to a stable one.
Thus, although "smart" materials are constantly being developed to make materials more responsive to external stimuli, more and more such materials are needed for a wide variety of applications. One such material that would be useful would be a "smart" pressure sensitive adhesive (PSA). PSAs having a nonplanar or structured surface are known, however, the structure created, either by coating particles or by microreplication techniques, remains through the life of the PSA without predetermined changes. Thus, it would be desirable to have a PSA with the ability to change shapes (e.g., surface structure) on demand with an increase in temperature to increase their breadth of applications. A PSA with these characteristics could be called a thermomorphic PSA, the shape or structure of which could be changed on demand.